One of the oldest and best-known devices used to store energy is the lead-acid battery. The technology is based upon the reduction of lead dioxide to lead sulfate at the positive electrode and the simultaneous oxidation of lead to lead sulfate at the negative electrode. The electrolyte, sulfuric acid, is consumed, and energy is discharged during this process. Energy is stored by reversing these reactions; that is, charging the battery.
The energy stored is proportional to the discharge voltage, which is about 2.08 volts per cell, and to the amount of lead species. Lead has a high atomic weight and is an inherently inefficient chemical for battery energy storage. Lead is also used for carrying current within the cell. As a result, the amount of lead required per kilowatt hour of storage may be 50 to 100 pounds, which unfavorably impacts both weight and cost of the storage system. This translates to an energy density of about 10 to 15 watt hours per pound, depending upon battery design, operation, and desired life.
Another way of looking at this low specific energy, is in regard to the high equivalent weights of the starting materials, which is only partially made up for by the very high cell voltage. Another important reason, however, is that very little of the charged, active material is available for discharge. Both positive and negative electrodes typically yield only 20-35% utilization. Anything higher than 60% is a laboratory curiosity. By contrast, both electrodes in a nickel-cadmium cell yield at least 70-80% and sometimes more. Lithium electrodes often approach 100%. Invariably, electrodes in other batteries exhibit utilization efficiencies well beyond the range of those in lead-acid batteries.
Problems relating to the efficiency of usage of batteries come from the reactions occurring within the batteries themselves. During discharge, the lead dioxide which is a fairly good conductor, from the positive plate is converted to lead sulfate, an insulator. The lead sulfate can form an impervious layer encapsulating the lead dioxide particles, thus partially eliminating access of the electrolyte to the active material.
In the case of a standard lead-acid battery, such batteries are easily and commercially available in charged form. By providing a plate made of lead dioxide (PbO.sub.2), in the presence of a lead (Pb) plate and sulfuric acid, a charged battery is formed which can then be discharged, and re-charged.
The use of pastes applied to the electrodes has been practiced to increase the availability of the reacting materials. The vertical positive and negative plates of lead based alloy grids have been coated with layers of electrochemically active pastes. In a charged condition, the paste on the positive electrode contains lead dioxide which is the positive active material, and the negative electrodes contain a sponge lead. Typically batteries are formed where both electrodes are lead and act only as conductors, but where the paste alone acts as the electrode material which undergoes chemical change.
One conceivable way to form the Pb/PbO.sub.2 battery is to start with the components which would be present in a discharged battery, and apply a charge to form the Pb/PbO.sub.2 starting structures. With respect to the negative electrode, and the use of metallic lead, the sponge lead forms fairly easily. However, the PbO.sub.2 does not form as readily. In some cases PbO, lead monoxide, interspersed with some free lead can help begin to build the starting structure during the charging reaction. The PbO will form the first PbO.sub.2, to be followed by further PbO.sub.2 formation. The theory behind the formation of the battery in this manner is that the formed Pb/PbO.sub.2 starting structures will have an expanded structure, and therefore be more amenable to reaction during discharge.
The use of pastes as a starting material is particularly useful for the bipolar battery which may be formed by a series of stacked lead plates, typically with one side having the positive paste and the other side having the negative paste. An ion permeable separator typically separates the plates, and the plates themselves form the electrical interconnection between the cells. The use of pastes, although increasing the active species and the overall surface area and availability of the active species, has had no significant effect in minimizing the mechanism described above which limits utilization of the electrodes.
The pastes which have been used today have typically been a "leady" oxide paste, such as PbO interspersed with some elemental free lead. This paste is typically applied to the grids of conventional lead-acid batteries, the grids containing the active ingredients, namely PbO.sub.2 are pasted with a "leady oxide" which is primarily PbO containing a small amount of free lead. This is for the purpose of assisting in the charging reaction, as was previously discussed. This contrasts with other battery formulations, such as where two differing pastes are utilized as the beginning of formation of the reactive elements themselves during the charging reaction.
It would be advantageous to begin with an expanded lead dioxide PbO.sub.2 as the positive electrode and an expanded sponge lead for the negative electrode. However, obtaining such expansion, without building the battery in reverse through the charging reaction is either impossible or impractical.
Either with or without the conventional use of PbO, the power output of the battery, even in the presence of such pastes, is significantly influenced by its state of discharge. Further, the lead sulfate, especially near the positive electrode can grow into large, hard, angular crystals, which can dislodge active material from the electrodes, and can further impede the reduction to lead at the negative electrode.
A beginning to the solution to the problem of under utilization of batteries was described in U.S. Pat. No. 4,507,372, issued on Mar. 26, 1985 to John J. Rowlette, inventor herein, and entitled "Positive Battery Plate," and which is incorporated herein by reference. The solution outlined in the `372 patent involved the addition of a conductive filler to the pastes. The filler of choice was stated to be tin dioxide, for the positive electrode, rather than the previously used graphite which was noted to undergo a reaction in the presence of the electrolyte to form acetic acid, and then to form lead acetate.
The use of tin dioxide is outlined in U.S. Pat. No. 4,713,306 to Pinsky et al, issued on Dec. 15, 1987 and entitled "Battery Element and Battery Incorporating Doped Tin Oxide Coated Substrate. The substance of this patent is incorporated herein by reference.
The addition of the tin dioxide can be accomplished as a pre-dispersed paste, or as a powder or coating onto a particulate or fibrous substrate, such as glass powder or glass wool. Further, the conductivity of the tin dioxide is greater than that of graphite. Stannic oxide additive is commercially available from Crystal Research of Olympia, Wash.
One of the most important reasons for the limitation in discharge of the active materials of a lead-acid cell is lack of access of the electrolyte to all parts of the active material. When the PbO.sub.2 in the positive plate discharges and becomes PbSO.sub.4 there is a theoretical volume expansion, based upon the dry compounds, from 25.15 cm.sup.3 /mole to 48.2 cm.sup.3 /mole. Assuming this volume expansion in the liquid phase to nearly approximate the volume differential in the solid phase from which these numbers were drawn, it can be seen that the reaction, and its accompanying volume expansion will eventually plug up the pores of the PbO.sub.2 and thus prevent the electrolyte from contacting the remaining particles of PbO.sub.2, making those remaining particles of PbO.sub.2 essentially inaccessible.
A similar problem occurs at the negative electrode, where the volume change from elemental Pb to PbSO.sub.4, based upon the dry preparations is from 18.25 cm.sup.3 /mole to 48.2 cm.sup.3 /mole. Of course, more utilization can always be achieved if the discharge rate is reduced, since a reduced discharge rate will not cause as rapid plugging and will allow for a more even, orderly expansion. However, such a reduced discharge rate limits the uses of the battery and will not satisfactorily be useful for higher useful discharge rates.
What is therefore needed is a battery composition which permits formation of the electrode materials in an expanded form and which will enable a higher usage for a given amount of current flow. The battery composition should be such that the reactive materials are stable, and that the battery is re-chargeable without a serious reduction in the utilization.